This application claims the benefit, under 35 U.S.C. § 365 of International Application PCT/US03/024351, filed Aug. 4, 2003, which was published in accordance with PCT Article 21(2) on Feb. 24, 2005 in English.
The present invention generally relates to wireless receiving devices, and more particularly, to user equipment (UE) in a spread-spectrum based wireless system such as the Universal Mobile Telephone System (UMTS).
The basic unit of time in UMTS radio signals is a 10 milli-second (ms) radio frame, which is divided into 15 slots of 2560 chips each. UMTS radio signals from a cell (or base station) to a UMTS receiver are “downlink signals,” while radio signals in the reverse direction are termed “uplink signals.” When a UMTS receiver is first turned on, the UMTS receiver performs a “cell search” to search for a cell to communicate with. In particular, and as described below, the UMTS receiver initially looks for a downlink synchronization channel (SCH) transmitted from the cell to synchronize thereto at the slot and frame levels, and to determine the particular scrambling code group of the cell. Only after a successful cell search can voice/data communications begin.
With respect to the cell search, the SCH is a sparse downlink channel that is only active during the first 256 chips of each slot. The SCH is made up of two subchannels, the Primary SCH (PSCH) and the Secondary SCH (SSCH). The PSCH 256 chip sequence, or PSCH code, is the same in all slots of the SCH for all cells. In contrast, the SSCH 256 chip sequence, or SSCH code, may be different in each of the 15 slots of a radio frame and is used to identify one of 64 possible scrambling code groups. In other words, each radio frame of the SCH repeats a scrambling code group sequence associated with the respective transmitting cell. Each SSCH code is taken from an alphabet of 16 possible SSCH codes.
As part of the cell search, the UMTS receiver first uses the PSCH to achieve slot synchronization. In this regard, the UMTS receiver correlates received samples of the received PSCH against the known PSCH 256 chip sequence (which is the same for all slots) and, based on the location of the correlation peak, determines a slot reference time. Once the slot reference time is determined, the UMTS receiver is slot synchronized and can determine when each slot starts in a received radio frame.
After slot synchronization, the UMTS receiver ceases processing of the PSCH and begins processing the SSCH. In particular, the UMTS receiver correlates the particular sequence of 15 SSCH codes in a received radio frame against known sequences to achieve frame synchronization and to determine the scrambling code group of the cell. Identification of the scrambling code group then enables the UMTS receiver to descramble all of the other downlink channels of the cell (e.g., the Common Pilot Channel (CPICH)) for voice/data communications to begin.
Unfortunately, the above-described cell search process has some drawbacks. One is time. Since SSCH processing involves the identification of a sequence of 15 particular SSCH codes, the SSCH code processing typically occurs over a number of received radio frames, e.g., 10 to 20. Therefore, completion of the cell search may take on the order of 100 to 200 ms. Another drawback is that the UMTS receiver does not achieve frequency synchronization until the CPICH is descrambled, which, as noted above, occurs after successful completion of the above-mentioned cell search. As such, and especially since the UMTS receiver may be mobile, it is possible that channel conditions may change during SSCH processing such that the UMTS receiver loses slot synchronization (e.g., the correlation peak moves or disappears). If this happens, the SSCH processing may fail. Further, this failure is not detected by the UMTS receiver until the SSCH processing finishes. Thus, the entire Cell Search process must start over—further lengthening the time a user must wait till voice/data communications can start.